COMPARISON OF FACTORS INFLUENCING ACIDITY OF HYDROXYL-GROUPS IN ZEOLITES - QUANTUM CHEMICAL STUDY

The nonempirical quantum chemical method with a STO-3G basis set is employed to study the effect of changes in the chemical composition of the environment of the →Si-OH-Al← fragment in zeolites on the acidity of the bridging OH groups modeled by the H-OX-TH2-O-SiH2-OH-AlH3 molecular model (T = Si or Al and X = H or Na). It is shown that the substitution of Si by Al (i.e., variation in the Si/Al ratio) is the most important factor influencing the OH group acidity. Its effect on the acidity is approximately twice as great as the effect of substitution of H by Na (i.e., variation in the degree of decationization). The bridging OH groups in immediate proximity to a terminal OH group were calculated to be somewhat more acidic than the OH groups surrounded by only Si or Al atoms. Neutral oxide-like species of metals occluded in the zeolite pores were found to increase the acidity of the adjacent OH groups. © 1990 American Chemical Society.